1. Field of the Invention
The present invention relates to a non-aqueous electrolyte capable of providing a lithium secondary battery having superior battery cycle characteristic and battery characteristics such as electrical capacity, storage characteristic, and also relates to a lithium secondary battery using the same.
2. Description of the Related Art
In recent years, lithium secondary batteries have been widely used as the power sources for driving compact electronic devices etc. Lithium secondary batteries are mainly composed of a cathode, a non-aqueous electrolyte and an anode. In particular, a lithium secondary battery having a lithium complex oxide such as LiCoO2 as a cathode and a carbonaceous material or lithium metal as an anode is suitably used. Further, as the non-aqueous electrolyte for a lithium secondary battery, a composition comprising a combination of a cyclic carbonate such as ethylene carbonate (EC) or propylene carbonate (PC) and a linear carbonate such as dimethyl carbonate (DMC), methylethyl carbonate (MEC), and diethyl carbonate (DEC) is suitably used.
However, a secondary battery having more superior battery cycle characteristic and battery characteristics such as electrical capacity has been desired. A lithium secondary battery using a highly crystallized carbonaceous material such as natural graphite or artificial graphite as the anode sometimes suffer from breakdown of the electrolyte at the anode and an increase in the irreversible capacity or in some cases peeling of the carboneous material occur. The increase in the irreversible capacity or the peeling of the carbonaceous material occurs due to the decomposition of the solvent in the electrolyte during the charge thereof and is due to the electrochemical reduction of the solvent at the interface between the carbonaceous material and the electrolyte. In particular, PC having a low melting point and high dielectric constant has a high electroconductivity even at a low temperature. Nevertheless, when a graphite anode is used, there are problems that the PC cannot be used for the lithium secondary battery due to the decomposition thereof. Further, EC partially decomposes during the repeated charge and discharge thereof so that the battery performance is decreased. Therefore, the battery cycle characteristic and the battery characteristics such as electrical capacity are not necessarily satisfied.
On the other hand, as the salt dissolved in the non-aqueous solvent, a lithium salt such as LiClO4, LiPF6 or LiBF4 is used. A non-aqueous electrolyte containing such a non-aqueous solvent and the LiPF6 dissolved therein is known to be high conductivity and high in the oxidation decomposition voltage of the LiPF6, and therefore, is stable at high voltage.
However, LiPF6 is inferior in heat stability, and therefore, there is the problem that the lithium salt is decomposed at a high temperature environment of 60° C. or more and the battery performances such as the cycle life under a high temperature environment are tremendously decreased. On the other hand, LiBF4, which is superior to LiPF6 in the heat stability, may be mentioned, but the ion conductivity that is inferior to that of LiPF6. Thus, there is the problem that battery performance such as the cycle life is decreased under an ordinary temperature environment. Therefore, a cyclic ester such as γ-butyrolactone (GBL) is used due to the relatively high conductivity thereof. However, when GBL is used for a lithium secondary battery using a highly crystallized carbonaceous material such as natural graphite or artificial graphite as an anode, the GBL will electrochemically be decomposed at the graphite anode interface at the time of charging, and therefore, the battery performance will be decreased along with repeated use of charging and discharging. Thus, at the present time, the battery cycle characteristic and battery characteristics are not necessarily satisfactory.